How to define endpoint to operate "normally" or use SSE for progress based on Accept header

[maxott]

How would I define a service endpoint POST /jobs which either waits until a job is done and returns the result or uses SSE to push progress events via SSE depending on the Accept header.

I did ask the friendly AI agent but didn't get an answer which would work ;)

Would this work?

    Method("create", func() {
        ....

        // Synchronous response
        Result(JobResult)
        // SSE streaming response
        StreamingResult(JobProgress)

        HTTP(func() {
            POST("/jobs")
            Response(StatusOK, func() {
                ContentType("application/json")
            })
            Response("streaming", StatusOK, func() {
                ContentType("text/event-stream")
            })
        })
    })
})

Or would I need to use SkipResponseBodyEncodeDecode and build my own reply? Also, will that create the correct OpenAPI spec which would allow client side code generators to work that out?

[raphael]

Yes — the Goa “mixed unary + SSE” pattern is to define both Result and StreamingResult with different types and enable SSE on the HTTP endpoint.

Something like:

Method("create", func() {
	Payload(JobCreatePayload)
	Result(JobResult)            // unary JSON
	StreamingResult(JobProgress) // SSE events

	HTTP(func() {
		POST("/jobs")
		ServerSentEvents()
		Response(StatusOK)
	})
})

• With Accept: text/event-stream, the transport negotiates SSE and your service implementation can stream progress events.
• With a normal Accept (e.g. application/json), the same method returns the unary JSON JobResult.
• You do not need SkipResponseBodyEncodeDecode for this.

This used to trigger a runtime panic / broken generated code in some cases; it’s fixed in PR #3883: #3883 (and the generated Go clients expose both unary and streaming variants for mixed-result methods).

[maxott]

Thanks for the reply.

I tried this and I'm wondering on how to differentiate in the implementation. The generated signature is:

Create(context.Context, *sapi.CreatePayload, sapi.CreateServerStream) (err error)

where CreateServerStream seems to represent the streaming scenario, but how to return the "normal" answer?

I'm seeing that the referenced PR is very recent. Is that already in a release, or should I wait until is being released?